This invention relates to a thermal transfer printer for printing on the surface of a compact disk and other media using a linear thermal transfer head.
The invention optimizes printing on irregularly shaped media and incorporates features to prevent damage to the thermal transfer print head and to facilitate ease of use and maintenance.
Compact disks are an inexpensive medium for storing digital information that may relate to audio, video and/or any type of information or data that is conveniently stored in digital form. When compact disks are manufactured in large quantities, the side opposite the recording side of the disk is customarily printed in a mass printing process such as silk screening. The label information applied to the disks is generally identical for each disk and related to the pre-recorded content of the disks.
With the development of the CD-R disk, disks can be sold in blank with the informational content later recorded by a CD-R recorder. In order to appropriately label such disks with regard to the content that is recorded on the disk, programmable disk printers, such as ink jet printers and thermal transfer printers have been devised. These printers print the surface of the disk with graphics and other information that can be customized to correspond to the information recorded on the disk by the CD-R recorder. One drawback in using an ink jet printer is the extended time required to print an individual disk. Another drawback is the additional expense of disk blanks, which require a precoated surface for ink jet printing. Additionally, use of inks that are water resistant is difficult and expensive requiring specialty print heads.
Thermal transfer printers can print with greater speed and print on disks prepared with an inexpensive lacquer coating. Thermal transfer printers include a print head that applies a contact pressure to the media to be printed.
One type of thermal transfer printer will typically consist of a mechanism that has a stationary print head, a ribbon, and assembly that moves the media under the print head. The print head contains an array of heating elements. The ribbon is a plastic film with a wax or resin compound deposited on one side. The print head is in contact with the ribbon during printing, and the ribbon is in contact with the media.
By heating the areas of the ribbon, the was or resin compound is deposited on the media. Printing occurs by moving ribbon and the media at the same rate across the print head, while firing the heating elements in a desired pattern. The print head must exert some pressure on the media for successful transfer of the wax or resin to the media.
A second type of thermal printer is a direct transfer printer, which uses thermally sensitive media that changes color when heated, therefore a ribbon is not required. With thermally sensitive media, the print head marks the media by generating a pattern of heated and non-heated areas on the surface of the media, as it moves under the print head. The invention described is applicable to both types of thermal printers.
Thermal transfer printers require the print head to contact the printable surface at a uniform pressure for optimum transfer of a marking medium from a ribbon to the media (or heat in the case of direct thermal transfer printer). Variations in print head pressure to the media result in improper printing on media such as non-printed areas or uneven print density.
Printing on rectangular objects, such as a piece of paper, is relatively straight forward, since the print head pressure remains constant during the entire printing process. The pressure remains constant because the area of contact between the print head and the media does not change. For example, in printing a 5xe2x80x3 wide piece of paper the print head is always in contact with 5xe2x80x3 of media. In contrast, printing on a 5xe2x80x3 diameter disk, the area of contact would initially be very small as the print head is at the edge of the disk, but then increases to 5xe2x80x3 as the print head crosses the center of the disk. After crossing the center of the disk, the area of contact decreases as the print head travels the far edge of the disk.
When the force of the print head applied to the media is constant and the print head travels across a rectangular shaped media, the pressure per unit area is constant. If the print head travels across a disk shaped media, the print head pressure to the media will change as the print head travels across the disk. When the force of the print head applied to the media is constant and the print head travels across a disk shaped media the pressure per unit area changes as the contact area increases and decreases.
To successfully print on disk shaped media, the printer must be constructed to either:
a) vary the force of the print head applied to the media as it travels across the disk to compensate for the variation in width of printable surface, or
b) hold the disk in a manner that effectively presents an unchanging width of contact area for the print head as it moves across the disk.
The process described in point a) can be achieved by using a complicated system of cams, gears and sensors.
The process described in point b) can be achieved by using a simple system based on the invention that incorporates a media holding tray that puts the print head in contact with the media and a supplemental surface. The combination of the surfaces which are in contact with the print head present a surface of uniform width (width that does not change as the disk is printed). This supplemental surface comprises a mask that has a thickness and structural characteristics that are substantially the same as the media.
The invention described below consists of a thermal printer that utilizes a tray type of media holder with materials arranged in such a manner as to maintain a uniform print head pressure to media as the media moves relative to the print head.
The media to be printed is placed manually or robotically in the media tray which consists of a base layer of compressible material (mounted on either a platform or platen) and a second mask layer of material similar to the thickness and composition of the media. The mask layer has a cutout in which the media is positioned. This arrangement allows the printable surface of the media to be at the same level as the unmasked areas of the compressible surface.
The key feature of this arrangement is that as the print head passes over the media, the area of contact between the print head and the sum of the areas of the media and the surface of the media holder remains constant. This results in uniform (unchanging) print head pressure on the media during the entire printing process.
By careful selection of the materials of the media holder, the proper print head to media pressure can be maintained without the use of complex print head pressure control systems. In addition, proper print head pressure can be maintained when printing odd shaped, non-rectangular media, such as disk shaped objects, where the print head""s area of contact with the media varies as the print head moves relative to the disk.
The base layer (compressible surface) and the mask layer (surface with cutout area in the shape of the media) may have one or more layers of material, so long as the surface of the mask layer has similar mechanical characteristics to the item being printed.
A typical composition of the base layer would consist of a material that compresses to the appropriate degree needed to maintain proper print head pressure distribution on the media. The preferred embodiment for the disk printing application would require a base layer material that has a compression value of 40-70 durometer which could include materials such as neoprene and other rubber-like substances.
A typical configuration of the mask layer would consist of a material that does not compress or has the same compression characteristics as the media. The preferred material for the mask layer of the disk printing application is a non-compressible material such as polycarbonate. CD-ROM and CD-R disks are typically made from molded polycarbonate.
The thermal printer of this invention is designed to print on various configurations of media, and in particular, on disk shaped media, such as a compact disk. The invented printer resolves the problem of printing with a uniform pressure across irregular shaped media. The unique features of this invention include a print head chassis that flips open for easy replacement of the print transfer ribbon and maintenance and servicing of the print head and internal components. Additionally, the improved thermal printer includes a replaceable media transport carrier in the form of a removable tray.
The thermal transfer printer of this invention includes a rigid carrier having a flat media support surface with a resilient base layer and a top mask layer. The top mask layer has a media mask with a cutout having a configurations that matches the configuration of the media item to be printed. The media mask is fabricated from a material having physical and structural characteristics that are substantially the same as the media item being printed. Additionally, the media mask has a thickness that matches the thickness of the media item. In certain applications, the media item may require a topographically tailored media support surface. The feature of the removable support tray permits a variety of trays with different tray templates to be provided including trays with custom, multi-level, complex support configurations to optimize print transfer.
In this manner, the thermal contact element in the print head of the thermal transfer printer distributes its contact force across both the media item and the mask. The resulting pressure per unit area applied to the media item thereby remains constant during each advance of the carrier relative to the contact edge of the print head.
Additionally, the thermal transfer printer of this invention includes an improved retaining mechanism to retain a media item in position during the printing process. The retaining mechanism is designed to avoid damage to the fragile thermal resistors forming the linear array of pixel generating elements in the contact edge of the print head.
The retaining mechanism includes a centrally located retainer that is activated to hold the media item against the edge of the media mask. The retainer is part of a retainer mechanism that is incorporated into the removable tray and is connected to a latching mechanism in the printer for operation. In the case of a compact disk having a circular perimeter, the mask includes two small edge protuberances that project into the complimentary circular shaped cutout area of the mask layer opposite the retainer. The retainer, in the form of a button when used for compact disks is activated against the edge of the disk to urge the disk against the protuberances, thereby positioning the disk on the centerline between the protuberances.
This arrangement avoids the use of multiple contact pins that may damage the fragile pixel generating elements in the contact edge of the thermal print head. In the improved printer the retainer is positioned at the leading edge of the disk which is printed as the tray is retracted into the printer. With this system, the printer is able to place the contact edge of the print head at the leading edge of the disk just behind the single disk holding button. This allows the disk to be printed with no chance of collision between the media holding retainer and the print head.
The invented transfer printer also includes a mechanism to detect the carrier position and detect whether a media item is properly positioned on the carrier before contact by the print head. The detection mechanism is incorporated into the improved actuatable retainer mechanism to hold the media item in place during printing. Other embodiments of a retainer include a shuttle bar for straight edge media such as truncated disks and rectangular recordable media popular for business cards, specialty jigs for tags and other printable items collectively defined as media items as described herein. These and other features are described in greater detail in the detailed description of the preferred embodiments that follows.